One way to improve the spectral efficiency and bitrate in a wireless network is to use Multiple-Input Multiple-Output (MIMO) technology. MIMO technology has been introduced in the Third Generation Partnership Project (3GPP) Release 7, and involves sending and receiving multiple information streams using multiple antennas at both the sending and receiving end.
Current implementations of MIMO require so-called precoding. Precoding allows the mapping of information streams to different antennas via a set of complex weights that are known to the receiver. In open-loop precoding, the precoding weights are predefined. In closed-loop precoding, the receiver estimates which precoding weights will give the best throughput, and signals this choice back to the transmitter. To reduce the need for feedback signalling, many standards specify a restricted set of possible precoder weights. The restricted set of possible precoder weights is known as the codebook. By using the codebook, the receiver can simply signal back the identity of the preferred precoder obtained from the codebook. The transmitter, which also has access to the codebook, can then apply the correct precoder weights to the transmitted MIMO signal.
In the Long Term Evolution (LTE) standard, closed-loop precoding may either be wideband or frequency-selective. Wideband precoding is more robust and requires less feedback information, but frequency selective precoding can better handle the case when the channel is frequency selective and no single precoder is optimal over the whole frequency band.
Transmitted information streams are typically mapped to orthogonal precoders at the transmitter. However, these streams, when transmitted over the wireless channel, will typically be mixed. This can best be understood by considering a simple orthogonal mapping consisting of transmitting stream 1 from transmit antenna 1 and stream 2 from transmit antenna 2. A receiver with two antennas will receive the superposition of the two transmitted streams on each of its receive antennas. The receiver must then separate the mixed streams. An optimal receiver is the so called Maximum Likelihood (ML) receiver. Such a receiver uses complex non-linear operations. A more practical receiver, that is more likely to be implemented in products, is the linear Minimum Mean Square Error (MMSE) receiver. An MMSE receiver is more sensitive to correlation between mixed streams in a channel.
The gains from MIMO operation are usually the highest when the wireless channels from each transmitting antenna to each receiving antenna have similar power, and experience uncorrelated fast fading variations. If fading correlation and/or power imbalance appear between the antennas channels, the gain normally decreases in comparison, reducing the advantages of MIMO.
MIMO has primarily been utilized for the down-link in cellular networks, where the transmitter (a base station) typically utilizes multiple antennas that are deployed in close proximity, and which have essentially equal radiation patterns. However, some other scenarios are now gaining increased attention.
An important scenario of interest is when distributed antennas are utilized. An example of a use of distributed antennas is an indoor deployments where a distributed antenna system can be used to provide coverage in a cell where the path loss from a single transmit/receive antenna is too high to support adequate communication quality everywhere within the cell area. The base station serving the cell has one antenna port, and a combining/splitting network that distributes the signals from the antenna port to multiple physical antennas within the area. This ensures that the “effective” antenna is very large, and users in the area are likely to be close to at least one of the physical antennas. In this way, users all have access to the base station in a large cell, whereas if the base station had physical antennas in the same location, users might be out of range or “line of sight”, and not be able to communicate with the base station.
Distributed antenna systems are most common in indoor environments such as an office building, shopping mall, or airport terminal. However, a distributed antenna system may also be deployed in an outdoor environment if the intended cell shape is difficult to achieve using a single antenna. An example is an elongated cell along a railway line. The peak bit rate and spectral efficiency improvement available using MIMO technology is very desirable in indoor areas where users require similar bit rates to wireless LANs. A straightforward approach to a distributed antenna system is illustrated in FIG. 1. An indoor environment 1, such as an airport terminal, is provided with several co-located physical antennas 2, 3, 4, 5, 6. Each antenna group has one physical antenna (denoted by a black filled circle) connected to port 1 of the base station, and another physical antenna (denoted by an unfilled circle) connected to port 2 of the base station. This type of arrangement ensures wide coverage owing to the distributed groups of antennas, and using grouped antennas connected to different ports of the base station allows higher bitrates between the base station and a user device using MIMO. However, this approach can be costly.
It is possible for MIMO to be used on the uplink (from the user device to the base station), where the user device is able to transmit using multiple antennas. Owing to the limitations of RF and antenna design in compact user devices, it is likely that unequal power will be transmitted from the antennas at the user device, owing to differences in radiation patterns, antenna efficiencies, shielding by the user, and/or owing to the design of the power amplifiers (for example, if a secondary power amplifier with less output power is used for a secondary antenna).